This invention relates to improvements in controller circuits for an electric pump incorporated into braking circuits for vehicles and to an improved method of operating an electric pump.
It is known to provide a hydraulic braking circuit of a vehicle of the kind comprising a hydraulic circuit connecting a source of pressure to a piston of a brake caliper. The source of pressure may comprise a fluid reservoir which is charged by an electric pump. The pressurised fluid in the reservoir may be used as an alternative, or as assistance to, the increase in pressure that occurs when a driver presses a brake pedal of the vehicle to operate a master cylinder. The additional pressure supplied by the reservoir may be used to prime the brake circuit upon initial detection of a driver pressing the brake pedal, thereby reducing the time it takes to bring the brakes up to sufficient pressure to produce useful braking force. It may be used to provide emergency braking as well, for instance if a sensor detects that an emergency stop is required the pump may be used to create the pressure needed to apply the brakes independent from the operation of the brake pedal. In addition it may be used to provide electronic stability control where the brakes are applied in the event that the vehicle is losing traction.
The pressure in the circuit may be increased by an electric pump, and in the prior art electric pumps include motors that have been driven by high frequency pulse width modulation of the voltage applied to the pump. This enables the pressure provided by the pump to be precisely controlled during application of the brakes or during an assistance mode such as anti-lock brake operation of electronic stability control. The size of the motor used is typically chosen so that the maximum pressure that is needed during any of these operations can be attained with the motor operating at a nominal peak operating powers. For pressures below that peak the PWM modulation reduces the average voltage applied the motor, reducing the motor speed.
One important requirement from the pump is an ability to rapidly increase the pressure in the circuit at the start of a stop. The faster the pump can increase the pressure in the system the sooner there is enough pressure for the brakes to be applied and the sooner the vehicle can brought to a stop. This reduces braking distances and increases safety. The rate at which a pump can increase the pressure is generally determined by the swept volume of the pistons or vanes of the pump, and the peak power of the electric motor that is used to move the pistons or vanes. A larger pump needs a higher power motor to drive it. A more powerful pump and motor will be able to increase pressure of the fluid faster than a smaller one for a given peak input voltage, as the bigger motor will try to draw more current from the motor. However, larger motors are not desirable as they are more costly than smaller motors.
The power that can be drawn by the motor is limited in prior art systems by the voltage available from the battery. This is typically set at around 13-14 volts for a passenger vehicle such as a car or light van, and cannot be increased without a redesign of all of the other electrical components of the vehicle. The voltage may drop considerably lower than this, perhaps to less than 10 volts, in some high load circumstances. Using a more powerful motor will draw more current but this is often not desirable. Drawing more current not only increases the power consumption but increases the rate at which the battery will drain. This is particularly the case for luxury vehicles which may have a large number of other electrical systems on board such as air conditioning, heated seats, electric window heaters, audio systems and so on, which also make high demands for current. Too much current can cause the battery to run flat. Another possible problem that can occur with high current drain is that it can cause the wiring loom to overheat. This could be ameliorated by increasing the gauge of the wires in the loom, but this increases cost and weight.